The Back Cover: Cholesterol Sulfate in a Membrane

lexander Smondyrev, a postdoctoral research associate in Professor Max Berkowitz’s group at the University of North Carolina at Chapel Hill, is taking the first steps in determining why nature went to such lengths to produce cholesterol, which plays a significant role in biological membranes. The image shows a membrane bilayer with a high concentration of cholesterol sulfate, which is found, for example, in the outermost layer of the skin and the membranes of red blood cells. Initially, the components of this membrane were arranged such that cholesterol sulfate molecules (green, with the red sulfate group enlarged) alternated with phospholipids (gray), fatty molecules in the membrane. The researchers selected this arrangement to reduce electrostatic repulsion between negatively charged cholesterol sulfate molecules. Water (blue) surrounds the system, and sodium atoms (purple) ensure the system’s electroneutrality. See story on Determining Why Nature Needs Cholesterol.